1. Field of the Invention
The present invention relates to fibers comprising, as at least one component, a thermoplastic polyvinyl alcohol with good solubility in water, and to a melt-spinning method for them. The invention also relates to fibrous structures such as yarns, woven fabrics, knitted fabrics and others comprising the fibers, and to fibrous products as obtained by processing the fibrous structures with water. The invention further relates to non-woven fabrics comprising a thermoplastic polyvinyl alcohol with good solubility or good flushability (disintegratability into fibers) in water.
2. Description of the Related Art
Water-soluble fibers comprising polyvinyl alcohol (PVA) are known, which are produced, for example, 1) in a wet-spinning or dry-jet-wet-spinning method in which both the dope solvent and the solidifying medium are of aqueous systems; 2) a dry-spinning method in which the dope solvent is of an aqueous system; and 3) a wet-spinning or dry-jet-wet-spinning (that is, gel-spinning) method in which both the dope solvent and the solidifying medium are of non-aqueous solvent systems.
These water-soluble PVA fibers are used as staple or short-cut fibers for dry non-woven and spun yarns and also in the field of papermaking, etc., or are used as multi-filaments for woven fabrics and knitted fabrics. Of those, in particular, short-cut fibers soluble in hot water at 80 to 90xc2x0 C. hold an important place in the papermaking industry, serving as a fibrous binder therein; and multi-filaments are much used as the base fabric for chemical lace. To solve the recent problems with the environment, they are specifically noticed as biodegradable fibers favorable to the ecology.
However, in the conventional spinning methods mentioned above, high-speed spinning, for example, at a rate over 500 m/min is difficult, complicatedly modified cross-section fibers having a high degree of cross-section modification are difficult to produce, and specific equipment for recovering various solvents used in the spinning step is needed. Therefore, as compared with a melt-spinning method, the conventional spinning methods are much restrained in various aspects and therefore inevitably require specific care.
In ordinary spinning technology of removing the solvent from the substance having been spun out through a spinning nozzle to give fibers, the surface of each fiber obtained is seen to have fine hillocks and recesses such as longitudinal streaks or the like running thereon in the direction of the fiber axis, when magnified to the size of 2000 times or more. Such fine hillocks and recesses formed on the fiber surface will induce fibrillation, when rubbed against the guide and others in the subsequent steps after the spinning step, thereby causing one reason for failed appearance and even end breakage of spun fibers.
Some examples of producing PVA fibers through melt-spinning are known. For example, in Japanese Patent Laid-Open No. 152062/1975, proposed is a technique of producing crapy woven fabrics, which comprises melt-spinning PVA copolymerized with a minor olefin to be a sheath component and a hydrophobic polymer substance to be a core component in a bi-component fiber spinning manner to give core/sheath bi-component fibers, weaving the resulting fibers into a fabric, and processing the fabric in an aqueous solution to dissolve and remove the PVA copolymer component of the bi-component fibers constituting the fabric. In Japanese Patent Laid-Open No. 152063/1975, another technique of producing crapy woven fabric is proposed. In this, core/sheath bi-component fibers composed of a mixture of PVA and a plasticizer serving as the sheath component and a hydrophobic polymer substance serving as the core component are woven into a fabric, and the fabric is processed in an aqueous solution to dissolve and remove the sheath component of the fibers constituting the fabric. In Japanese Patent Laid-Open No. 105122/1988, proposed are bi-component fibers comprising a modified PVA as one component. In this, the modified PVA is dissolved and removed in the step of post-processing the fibers.
However, the prior art techniques noted above are still problematic in that the solubility of PVA in water is poor and the fibers being spun are often broken. It has heretofore been impossible to produce PVA fibers satisfying both the requirements of good solubility in water and good spinning process stability. On the other hand, in the technique of completely removing water-soluble fibers, for example, for producing chemical lace or spun yarn with hollow structure, single-component fibers of PVA alone but not bi-component fibers are used. For bi-component fibers comprising PVA, even when the fiber-forming capability of the water-soluble PVA used as one component is not good, spinning them is possible so far as the other polymer to be combined with PVA has the capability of forming fibers. However, for single-component fibers of PVA alone, PVA must have good capability of forming fibers by itself. Therefore, the problem in fiber spinning is that planning polymer constitution and settling spinning conditions are more difficult in single-component fiber spinning than in bi-component fiber spinning.
Non-woven fabric structures are often used for disposable fiber products, and non-woven fabrics comprising PVA fibers have been proposed for them. In some applications, those not completely soluble in water but capable of losing their non-woven texture to be disposable are used. However, for most non-woven fabrics of water-soluble PVA that have heretofore been proposed, PVA fibers are produced in a wet or dry-jet-wet spinning method. In Japanese Patent Laid-Open No. 345013/1993, partially proposed is a melt-spinning method for non-woven PVA fabrics. However, this has no concrete description of the method, and, needless-to-say, does neither disclose nor suggest what type of PVA shall be used for satisfying all the requirements of spinning process stability and solubility or flushability in water.
The present invention is to solve the problems with the conventional water-soluble PVA fibers noted above for their process stability and solubility in water, and its one object is to provide a stable melt-spinning method for fibers comprising a water-soluble polyvinyl alcohol as at least one component. Being different from the conventional wet-spinning, dry-jet-wet-spinning, dry-spinning and solvent-spinning methods noted above, the method provided herein is free from productivity limitation and cross-section profile limitation of the fibers produced, and does not require any specific equipment for product recovery.
Another object of the invention is to provide non-woven PVA fabrics with good solubility or good flushability (disintegratability into fibers) in water for which the PVA fibers are produced in a stable melt-spinning method but not in the conventional wet-spinning, dry-jet-wet-spinning, dry-spinning or solvent-spinning method.
Specifically, the invention provides thermoplastic polyvinyl alcohol fibers which comprise, as at least one component, a water-soluble polyvinyl alcohol containing from 0.1 to 25 mol % of C1-4 xcex1-olefin units and/or vinyl ether units, having a molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression of being from 70 to 99.9 mol %, having a carboxylic acid and lactone ring content of from 0.02 to 0.15 mol %, and having a melting point(Tm) falling between 160xc2x0 C. and 230xc2x0 C., and which contain an alkali metal ion in an amount in terms of sodium ion of 0.0003 to 1 part by weight based on 100 parts by weight of the polyvinyl alcohol.
The invention also provides a method for producing thermoplastic polyvinyl alcohol fibers, which comprises melt-spinning the polyvinyl alcohol noted above at a spinneret temperature falling between melting point(Tm) and Tm+80xc2x0 C., at a shear rate (xcex3) of from 1,000 to 25,000 secxe2x88x921, and at a draft of from 10 to 500.
The invention further provides a method for producing fibrous products, which comprises processing fibrous structures that contain, as at least one component, the thermoplastic polyvinyl alcohol fibers noted above with water to thereby dissolve and remove the polyvinyl alcohol.
The invention still further provides a non-woven fabric which is composed of fibers comprising, as at least one component, a modified polyvinyl alcohol containing from 0.1 to 25 mol % of C1-4 xcex1-olefin units and/or vinyl ether units, having a molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression of being from 66 to 99.9 mol %, having a carboxylic acid and lactone ring content of from 0.02 to 0.15 mol %, and having a melting point falling between 160xc2x0 C. and 230xc2x0 C., and which contains an alkali metal ion in an amount in terms of sodium ion of 0.0003 to 1 part by weight based on 100 parts by weight of the polyvinyl alcohol.
Polyvinyl alcohol for use in the invention is a modified polyvinyl alcohol with functional groups introduced thereinto through copolymerization, terminal modification and/or post-reaction, and it contains a specific amount of carboxylic acid and lactone ring moieties.
Methods for producing PVA with carboxylic acid and lactone ring moieties therein include, for example, the following:
{circle around (1)} A method of saponifying a vinyl ester polymer as obtained by copolymerizing a vinyl ester monomer such as vinyl acetate or the like with a monomer having the ability to form a carboxylic acid and a lactone ring, in an alcohol or dimethylsulfoxide solution.
{circle around (2)} A method of polymerizing a vinyl ester monomer in the presence of a carboxylic acid-having thiol compound such as mercaptoacetic acid, 3-mercaptopropionic acid or the like, followed by saponifying the resulting polymer.
{circle around (3)} A method of polymerizing a vinyl ester monomer such as vinyl acetate or the like along with chain transfer reaction on the alkyl group in the vinyl ester monomer and in the resulting vinyl ester polymer to give a high-branched vinyl ester polymer, followed by saponifying the polymer.
{circle around (4)} A method of reacting a copolymer of an epoxy group-having monomer and a vinyl ester monomer, with a carboxyl group-having thiol compound, followed by saponifying the resulting reaction product.
{circle around (5)} A method of acetalyzing PVA with a carboxyl group-having aldehyde.
The vinyl ester monomer includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, etc. Of those, preferred is vinyl acetate for producing PVA.
The monomer having the ability to produce a carboxylic acid and a lactone ring includes, for example, monomers having a carboxyl group derived from fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, etc.; acrylic acid and its salts; acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, etc.; methacrylic acid and its salts; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, etc.; acrylamide and its derivatives such as N-methylacrylamide, N-ethylacrylamide, etc.; methacrylamide and its derivatives such as N-methylmethacrylamide, N-ethylmethacrylamide, etc.
The comonomers that may be introduced into PVA in the invention include, for example, xcex1-olefins such as ethylene, propylene, 1-butene, isobutene, 1-hexene, etc.; acrylic acid and its salts; acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, etc.; methacrylic acid and its salts; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, etc.; acrylamide and its derivatives such as N-methylacrylamide, N-ethylacrylamide, etc.; methacrylamide and its derivatives such as N-methylmethacrylamide, N-ethylmethacrylamide, etc.; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, etc.; hydroxyl group-having vinyl ethers such as ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, 1,4-butanediol vinyl ether, etc.; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, hexyl allyl ether, etc.; oxyalkylene group-having monomers; vinylsilyls such as vinyltrimethoxysilane, etc.; hydroxyl group-having xcex1-olefins such as isopropenyl acetate, 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, 3-methyl-3-buten-1-ol, etc.;monomers having a carboxyl group derived from fumaric acid, maleic acid, itaconic acid, maleic anhydride, phthalic anhydride, trimellitic anhydride, itaconic anhydride, etc.; monomers having a sulfonic acid group derived from ethylenesulfonic acid, allylsulfonic acid, methallylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, etc.; monomers having a cationic group derived from vinyloxyethyltrimethylammonium chloride, vinyloxybutyltrimethylammonium chloride, vinyloxyethyldimethylamine, vinyloxymethyldiethylamine, N-acrylamidomethyltrimethylammonium chloride, N-acrylamidoethyltrimethylammonium chloride, N-acrylamidodimethylamine, allyltrimethylammonium chloride, methallyltrimethylammonium chloride, dimethylallylamine, allylethylamine, etc. The monomer content of PVA is at most 25 mol %.
Of those monomers, preferred are xcex1-olefins such as ethylene, propylene, 1-butene, isobutene, 1-hexene, etc.; vinyl ethers such a methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, etc.; hydroxyl group-having vinyl ethers such as ethylene glycol vinyl ether, 1,3-propanediol vinyl ether, 1,4-butanediol vinyl ether, etc.; allyl acetate; allyl ethers such as propyl allyl ether, butyl allyl ether, hexyl allyl ether, etc.; oxyalkylene group-having monomers; hydroxyl group-having xcex1-olefins such as 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol, 3-methyl-3-buten-1-ol, etc., as they are easily available.
More preferred are C1-4 xcex1-olefins such as ethylene, propylene, 1-butene, isobutene, etc.; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, etc., in view of their copolymerizability, of the melt-spinnability of PVA modified with them, and of the solubility in water of the PVA fibers. PVA contains from 0.1 to 25 mol %, but preferably from 4 to 15 mol %, more preferably from 6 to 13 mol % of the units derived from C1-4 xcex1-olefins and/or vinyl ethers.
Ethylene is preferred as the xcex1-olefin, as improving the physical properties of the PVA fibers. Therefore, it is especially preferable to use a modified PVA with from 4 to 15 mol %, more preferably from 6 to 13 mol % of ethylene units introduced therein.
PVA for use in the invention may be prepared in any known method of bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization or the like. Of those, generally employed is a bulk polymerization method or a solution polymerization method in which the monomers are polymerized in the absence of a solvent or in the presence of a solvent such as alcohol or the like. The alcohol used as the solvent for solution polymerization includes, for example, lower alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, etc. The initiator to be used for copolymerization may be any known one, including, for example, azo-type initiators and peroxide-type initiators such as xcex1, xcex1-azobisisobutyronitrile, 2,2xe2x80x2-azobis(2,4-dimethyl-valeronitrile), benzoyl peroxide, n-propyl peroxycarbonate, etc. The polymerization temperature may fall between 0xc2x0 C. and 150xc2x0 C. For PVA desired to be soluble in water at lower temperatures, the polymerization temperature is preferably not lower than 40xc2x0 C., more preferably not lower than 50xc2x0 C. However, if the polymerization temperature is too high, the degree of polymerization of PVA produced will be too low. Therefore, it is desirable that the polymerization temperature is not higher than 130xc2x0 C., more preferably not higher than 120xc2x0 C.
The molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression as referred to herein is meant to indicate the peak (I) for PVA as measured in d6-DMSO at 65xc2x0 C. with a 500 MHz proton NMR (JEOL GX-500 Model), which reflects the triad tacticity of the hydroxyl protons in PVA.
The peak (I) indicates the total sum of the hydroxyl groups in the vinyl alcohol units in PVA, appearing for the isotacticity chain (4.54 ppm), the heterotacticity chain (4.36 ppm) and the syndiotacticity chain (4.13 ppm) in triad expression; and the peak (II) appearing for all hydroxyl groups in the vinyl alcohol units in PVA is within the chemical shift region falling between 4.05 ppm and 4.70 ppm. Therefore, in the invention, the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is represented by: 100xc3x97(I)/(II).
In the invention, the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is controlled in the manner as specifically defined herein, whereby the water-related properties including solubility in water and water absorbability of PVA, the mechanical properties including strength, elongation and modulus of PVA fibers, and also the melt-spinning-related properties including melting point and melt viscosity of PVA are well controlled enough to meet the object of the invention. This is because a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is rich in crystallinity and could well exhibit the characteristics of PVA.
In the present invention, except for non-woven fabrics, the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression falls between 70 and 99.9 mol %, but preferably between 72 and 99 mol %, more preferably between 74 and 97 mol %, even more preferably between 75 and 96 mol %, further preferably between 76 and 95 mol %. In non-woven fabrics, PVA may not be completely dissolved for some applications so far as the fabrics can be disintegrated into fibers. In those, therefore, the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression may fall between 66 and 99.9 mol %, but preferably between 70 and 99 mol %, more preferably between 74 and 97 mol %, even more preferably between 75 and 96 mol %, further preferably between 76 and 95 mol %.
If the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is lower than the defined lowermost limit, the crystallinity of the polymer PVA is low. If so, the strength of the PVA fibers will be low, and, in addition, while the fibers are melt-spun, they will be glued together and the wound fibers could not be unwound. What is more, thermoplastic fibers having good solubility in water and also non-woven fabrics having good flushability, which the invention is intended to obtain, could not be obtained.
On the other hand, if the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is higher than 99.9 mol %, the melt-spinning temperature for the polymer PVA must be high since the melting point of the polymer is high. If so, the polymer being melt-spun will be decomposed, gelled or colored, as its heat stability is not good.
Where PVA for use in the invention is an ethylene-modified PVA, the PVA preferably satisfies the following formula, as producing better results.
xe2x88x921.5xc3x97Et+100 greater than molar fractionxe2x89xa7xe2x88x92Et+85
wherein the molar fraction indicates the molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression; and Et indicates the ethylene content (unit: mol %) of the PVA.
The carboxylic acid and lactone ring content of PVA for use in the invention falls between 0.02 and 0.15 mol %, but preferably between 0.022 and 0.145 mol %, more preferably between 0.024 and 0.13 mol %, even more preferably between 0.025 and 0.13 mol %. The carboxylic acid in the invention includes its alkali metal salts, and the alkali metal includes potassium, sodium, etc.
If the carboxylic acid and lactone ring content of PVA is smaller than 0.02 mol %, PVA greatly gels while it is melt-spun, and its melt-spinnability is poor. If so, in addition, the solubility in water of PVA is low. On the other hand, if the carboxylic acid and lactone ring content of PVA is larger than 0.15 mol %, the heat stability of PVA is poor. If so, PVA pyrolyzes and gels, and therefore could not be spun in melt.
The carboxylic acid and lactone ring content of PVA can be obtained from the peak appearing in proton NMR of PVA. Briefly, PVA is completely saponified to have a degree of saponification of at least 99.95 mol %, then fully washed with methanol, and thereafter dried in vacuum at 90xc2x0 C. for 2 days to prepare a sample of PVA to be analyzed through proton NMR.
Concretely, in the method {circle around (1)} mentioned above, the PVA sample prepared is dissolved in DMSO-D6, and subjected to 500 MHz proton NMR (with JEOL GX-500) at 60xc2x0 C. The content of the monomers of acrylic acid, acrylates, acrylamide and acrylamide derivatives constituting the polymer PVA is calculated in an ordinary manner from the peak (2.0 ppm) derived from the main chain methine of the polymer; and that of the monomers of methacrylic acid, methacrylates, methacrylamide and methacrylamide derivatives constituting it is from the peaks (0.6 to 1.1 ppm) derived from the methyl groups directly bonding to the main chain of the polymer. To measure the content of the monomers having a carboxyl group derived from fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride or the like, the PVA sample prepared is dissolved in DMSO-D6, to which is added a few drops of trifluoroacetic acid, and the resulting PVA solution is subjected to 500 MHz proton NMR (with JEOL GX-500) at 60xc2x0 C. The monomer content is calculated in an ordinary manner, based on the methine peak for the lactone ring assigned to the region between 4.6 and 5.2 ppm.
For PVA prepared in the methods {circle around (2)} and {circle around (4)}, the monomer content is calculated, based on the peak (2.8 ppm) derived from the methylene directly bonding to the sulfur atom.
In the method {circle around (1)}, the PVA sample prepared is dissolved in methanol-D4/D2O=2/8, and the resulting solution is subjected to 500 MHz proton NMR (with JEOL GX-500) at 80xc2x0 C. The methylene-derived peaks for the terminal carboxylic acid or its alkali metal salt (see the following structural formula 1 and structural formula 2) are assigned to 2.2 ppm (integrated value A) and to 2.3 ppm (integrated value B); the methylene-derived peak for the terminal lactone ring (see the following structural formula 3) is to 2.6 ppm (integrated value C); and the methine-derived peaks for the vinyl alcohol units are to the region falling between 3.5 and 4.15 ppm (integrated value D). The carboxylic acid and lactone ring content of PVA is calculated, as in the following formula in which xcex94 indicates the degree of modification (mol %).
Carboxylic acid and lactone content (mol %)=50xc3x97(A+B+C)xc3x97{(100xe2x88x92xcex94)/(100xc3x97D)}xc3x97100xe2x80x83xe2x80x83(3).
(Na)HOOCCH2CH2CH2xcx9cxe2x80x83xe2x80x83Structural Formula 1:
NaOOCCH2CH2CH(OH)xcx9cxe2x80x83xe2x80x83Structural Formula 2:

In the method {circle around (2)}, the PVA sample prepared is dissolved in DMSO-D6, and the resulting solution is subjected to 500 MHz proton NMR (with JEOL GX-500) at 60xc2x0 C. Based on the peaks derived from the methine group in the acetal moieties and appearing within the region between 4.8 and 5.2 ppm (see the following structural formula 4), the monomer content is calculated in an ordinary manner. 
wherein X indicates single bond or an alkyl group having from 1 to 10 carbon atoms.
PVA for use in the invention has a melting point(Tm) falling between 160 and 230xc2x0 C., preferably between 170 and 227xc2x0 C., more preferably between 175 and 224xc2x0 C., even more preferably between 180 and 220xc2x0 C. PVA having a melting point of lower than 160xc2x0 C. has poor crystallinity, and the strength of its fibers is poor. As the case may be, in addition, it could not form fibers as its heat stability is poor. What is more, when the PVA fibers are melt-blown, the resulting web will have many resin beads(shot) and could not keep its properties, or, as the case may be, they could not form web.
On the other hand, PVA having a melting point of higher than 230xc2x0 C. must be melt-spun at high temperatures. That is, the melt-spinning temperature for it will be near to its decomposition point. As a result, stably processing it to form fibers or to form non-woven fabrics in melt-blowing will be impossible.
The melting point of PVA may be measured through DSC (with Mettler""s TA3000). Briefly, using the DSC device, a sample of PVA to be measured is heated up to 250xc2x0 C. in nitrogen at a heating rate of 10xc2x0 C./min, then cooled to room temperature, and again heated up to 250xc2x0 C. at a heating rate of 10xc2x0 C./min. The top of the endothermic peak appearing in the heat cycle is read, and this indicates the melting point of the PVA.
The alkali metal ion content of PVA for use in the invention falls between 0.0003 and 1 part by weight in terms of sodium ion and relative to 100 parts by weight of PVA, but preferably between 0.0003 and 0.8 parts by weight, more preferably between 0.0005 and 0.6 parts by weight, even more preferably between 0.0005 and 0.5 parts by weight. If the alkali metal ion content of PVA is smaller than 0.0003 parts by weight, the PVA fibers are not sufficiently soluble in water. If so, the PVA fibers will give some insoluble in water. If, on the other hand, the alkali metal ion content of PVA is larger than 1 part by weight, PVA decomposes and gels too much while it is melt-spun, and could not form fibers.
The alkali metal ion includes, for example, ions of potassium, sodium, etc.
In the invention, the specific amount of an alkali metal ion is incorporated into PVA, for which the method is not specifically defined. For example, employable is a method of adding an alkali metal ion-having compound to PVA having been prepared through polymerization; or a method of saponifying a vinyl ester polymer in a solvent, in which an alkali metal ion-having, alkaline substance is used as the catalyst for saponification to thereby introduce the alkali metal ion into PVA, and the thus-saponified PVA is washed with a washing liquid so as to control the alkali metal ion content of the PVA. The latter is preferred.
The alkali metal ion content of PVA can be measured through atomic absorptiometry.
The alkaline substance to be used as the catalyst for saponification includes, for example, potassium hydroxide and sodium hydroxide. The molar ratio of the alkaline substance to be used as the catalyst for saponification to the vinyl acetate units in the polymer to be saponified preferably falls between 0.004 and 0.5, more preferably between 0.005 and 0.05. The catalyst for saponification may be added all at a time in the initial stage of saponification, or may be intermittently added in the course of saponification.
The solvent for saponification includes, for example, methanol, methyl acetate, diethyl sulfoxide, dimethylformamide, etc. Of those solvents, preferred is methanol; more preferred is methanol having a controlled water content of from 0.001 to 1% by weight; even more preferred is methanol having a controlled water content of from 0.003 to 0.9% by weight; and still more preferred is methanol having a controlled water content of from 0.005 to 0.8% by weight. The washing liquid includes, for example, methanol, acetone, methyl acetate, ethyl acetates, hexane, water, etc. Of those, preferred are methanol, methyl acetate and water, which may be used either singly or as combined.
The amount of the washing liquid is so controlled that the alkali metal ion content of PVA could fall within the defined range, but, in general, it falls preferably between 300 and 10000 parts by weight, more preferably between 500 and 5000 parts by weight, relative to 100 parts by weight of PVA. The washing temperature preferably falls between 5 and 80xc2x0 C., more preferably between 20 and 70xc2x0 C. The washing time preferably falls between 20 minutes and 10 hours, more preferably between 1 hour and 6 hours.
Preferably, the viscosity-average degree of polymerization (hereinafter simply referred to as the degree of polymerization) of PVA to be produced in the manner noted above falls between 200 and 500, more preferably between 230 and 470, even more preferably between 250 and 450. PVA having a degree of polymerization of smaller than 200 causes poor melt-spinnability, and it will often fail to form fibers. On the other hand, PVA having a degree of polymerization of larger than 500 will often fail to pass through a spinning nozzle, as its melt viscosity is too high. What is more, if PVA having such a high degree of polymerization is formed into a melt-blown non-woven fabric, the mean diameter of the fibers constituting the non-woven fabric will be large and the fibers will be partially coiled or rounded to form aggregates in the fabric. The non-woven fabric thus having such fiber aggregates will have a rough feel, and will often lose the characteristics intrinsic to melt-blown non-woven fabrics.
So-called low-polymerization PVA having a low degree of polymerization rapidly dissolve in an aqueous solution, and, in addition, the fibers comprising the PVA of that type will shrink to a reduced degree when they are processed in an aqueous solution to dissolve the PVA component therein.
The degree of polymerization (P) of PVA can be measured according to JIS-K6726. Briefly, PVA is re-saponified and then purified, and its limiting viscosity [xcex7] in water at 30xc2x0 C. is measured, from which the degree of polymerization, P, of PVA is obtained as in the following equation:
P=([xcex7]xc3x97103/8.29)(1/0.62).
PVA of which the degree of polymerization falls within the defined range as above produces better results.
Preferably, the degree of saponification of PVA falls between 90 and 99.99 mol %, preferably between 93 and 99.98 mol %, more preferably between 94 and 99.97 mol %, even more preferably between 96 and 99.96 mol %. PVA having a degree of saponification of smaller than 90 mol % could not be melt-spun in a satisfactory manner, as its heat stability is poor and it often pyrolyzes or gels. In addition, depending on the type of the comonomers constituting it, PVA having such a low degree of saponification will be poorly soluble in water and often could not attain the object of the invention.
On the other hand, it is impossible to stably produce PVA having a degree of saponification of larger than 99.99 mol %, and, even if produced, PVA of that type often fails to form stable fibers.
So far as they do not interfere with the object and the effect of the invention, various additives may be added to PVA in the course of polymerization to prepare PVA or during post-treatment of the polymer PVA. The optional additives include, for example, stabilizers such as copper compounds, etc., as well as colorants, UV absorbents, light stabilizers, antioxidants, antistatic agents, flame retardants, plasticizers, lubricants, crystallization retardants, etc. Adding heat stabilizers to PVA is preferred, as they improve the melt residence stability of PVA being formed into fibers. Preferred heat stabilizers include organic stabilizers such as hindered phenols, etc.; copper halides such as copper iodide, etc.; alkali metal halides such as potassium iodide, etc.
Also if desired, fine particles having a mean particle size of from 0.01 xcexcm to 5 xcexcm may be added to PVA, in an amount of from 0.05% by weight to 10% by weight, in the course of polymerization to prepare PVA or during post-treatment of the polymer PVA. The type of the fine particles is not specifically defined. For example, inert fine particles of silica, alumina, titanium oxide, calcium carbonate, barium sulfate or the like may be added thereto, either singly or as combined. Especially preferred are inorganic fine particles having a mean particle size of from 0.02 xcexcm to 1 xcexcm, as improving the spinnability and drawability of PVA.
The thermoplastic polyvinyl alcohol fibers of the invention include not only fibers of PVA alone but also multi-component spun fibers such as conjugate fibers and mixed spun fibers comprising PVA as one component, and some other thermoplastic polymers having a melting point of not higher than 270xc2x0 C. The combination pattern of each component in cross-section of multi-component fibers is not specifically defined, including, for example, core/sheath fibers, island/sea fibers, side-by-side fibers, multi-layered fibers, radial division fibers, and their combinations. For example, in bi-component fibers composed of PVA serving as the sea component and a different thermoplastic polymer serving as the island component, the sea component PVA may be removed to give ultra-fine fibers. In bi-component fibers composed of PVA serving as the core component and a different thermoplastic polymer serving as the sheath component, the core component PVA may be removed to give hollow fibers. A fabric of bi-component fibers composed of PVA serving as the sheath component and a different thermoplastic polymer serving as the core component may be processed with water to remove the sheath component PVA. After having been thus processed, the fabric could have an improved feel. On the other hand, the fabric of bi-component fibers of that type may be processed in a different manner of positively leaving the sheath component only as it is, and the remaining fibers could be useful as binder fibers.
In multi-component fibers, the polymers to be combined with PVA are preferably thermoplastic fibers having a melting point of not higher than 270xc2x0 C. For example, they include aromatic polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyhexamethylene terephthalate, etc., and their copolymers; aliphatic polyesters and their copolymers such as polylactic acid, polyethylene succinate, polybutylene succinate, polybutylene succinate adipate, polyhydroxybutyrate-polyhydroxyvalerate copolymers, polycaprolactone, etc.; aliphatic polyamides and their copolymers such as nylon 6, nylon 66, nylon 10, nylon 12, nylon 6-12, etc.; polyolefins such as polypropylene, polyethylene, polymethylpentene, etc., and their copolymers; modified polyvinyl alcohol having from 25 mol % to 70 mol % of ethylene units; as well as polystyrene elastomers, polydiene elastomers, chlorine-containing elastomers, polyolefin elastomers, polyester elastomers, polyurethane elastomers, polyamide elastomers, etc. At least one of these polymers may be combined with PVA to give multi-component fibers.
Of those, preferred are polybutylene terephthalate, ethylene terephthalate copolymers, polylactic acid, nylon 6, nylon 6-12, polypropylene, and modified polyvinyl alcohol having from 25 mol % to 70 molt of ethylene units, as being readily multi-spun with PVA for use in the invention.
For the polyester copolymers usable herein, the comonomers include, for example, aromatic dicarboxylic acids such as isophthalic acid, naphthalene-2,6-dicarboxylic acid, phthalic acid, xcex1,xcex2-(4-carboxyphenoxy)ethane, 4,4xe2x80x2-dicarboxydiphenyl, 5-sodium sulfoisophthalate, etc.; aliphatic dicarboxylic acids such as adipic acid, sebacic acid, etc.; and diol compounds such as diethylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, cyclohexane-1,4-dimethanol, polyethylene glycol, polytrimethylene glycol, polypropylene glycol, polytetramethylene glycol, etc. The proportion of the comonomers in copolymerization is preferably at most 80 mol %.
Where multi-component fibers comprising, as one component, an aliphatic polyester such as polylactic acid or the like are processed to remove the other component from them, thereby producing aliphatic polyester fibers, the aliphatic polyester fibers produced will be degraded or decomposed if the other component is removed through extraction with some chemicals except water. Therefore, in producing the multi-component fibers comprising such as polyester as one component, it is effective to use, as the other component, PVA shown herein for the present invention.
In producing bi-component fibers in the invention, it is preferable to use an aliphatic polyester such as polylactic acid or the like as the thermoplastic polymer having a melting point of not higher than 270xc2x0 C., since polylactic acid is biodegradable by itself and since the polyvinyl alcohol component having been removed from the fibers through extraction with water to be in an aqueous solution thereof is also biodegradable. As a whole, therefore, the bi-component fibers of that type are biodegradable.
In any mode of single-component spinning or multi-component spinning for producing the fibers of the invention, employable are any known melt-spinning devices. For example, in the mode of single-component spinning for producing them, PVA pellets are kneaded in melt in a melt extruder, then the resulting polymer melt is introduced into a spinning head, metered with a gear pump, and spun out through a spinning nozzle, and the thus-spun fibers are wound up. In the mode of multi-component spinning for producing multi-component fibers of the invention, PVA and other thermoplastic polymers are separately kneaded in melt in different extruders, and the resulting polymer melts are all spun out through one and the same spinning nozzle.
The cross-section profile of the fibers.is not limited to only a roundish one, but maybe C-shaped or may be poly-leafed, for example, 3-leafed, T-shaped, 4-leafed, 5-leafed, 6-leafed, 7-leafed or 8-leafed, or may also be cross-shaped.
In forming PVA into fibers in the invention, it is important that PVA is melt-spun at a spinneret temperature falling between Tm and Tm+80xc2x0 C., at a shear rate (xcex3) of from 1,000 to 25,000 secxe2x88x921, and at a draft, V, of from 10 to 500. Where PVA is melt-spun along with other polymers to give multi-component fibers, it is desirable that the melt viscosity of PVA and that of the other polymers to be combined with PVA are near to each other, when measured at the temperature of the spinneret through which they are melt-spun and at the shear rate at which they pass through the spinning nozzle, in view of the spinning stability of the combined polymer components.
The melting point, Tm, of PVA for use in the invention is the peak temperature for the main endothermic peak of PVA seen in differential scanning calorimetry (DSC, for example, with Mettler""s TA3000). The shear rate (xcex3) is represented by: xcex3=4Q/xcfx80r3 in which r (cm) indicates the nozzle radius, and Q (cm3/sec) indicates the polymer output rate per orifice. The draft, V, is represented by: V=Axc2x7xcfx80r2/Q in which A (m/min) indicate the take-up speed.
In producing the fibers of the invention, if the spinneret temperature is lower than the melting point, Tm, of PVA, PVA does not melt and therefore could not be spun. If, on the other hand, it is higher than Tm+80xc2x0 C., PVA will pyrolyze easily and its spinnability will become poor. If the shear rate is lower than 1,000 secxe2x88x921, the PVA fibers being spun will be readily broken; but if higher than 25,000 secxe2x88x921, the back pressure against the nozzle will be too high and the spinnability of PVA will be poor. If the draft is lower than 10, the fineness of the PVA fibers produced will be uneven and stable spinning of PVA is difficult; but if higher than 500, the PVA fibers being spun will be readily broken.
In the invention, adding a plasticizer to PVA to be spun is desirable, as improving spinnability of PVA.
The plasticizer is not specifically defined, and may be any compound having the ability to lower the glass transition point and the melt viscosity of PVA. For example, it includes water, ethylene glycol and its oligomer, polyethylene glycol, propylene glycol and its oligomer, butylene glycol and its oligomer, polyglycerin derivatives, glycerin derivatives as prepared by adding an alkylene oxide such as ethylene oxide, propylene oxide or the like to glycerin, sorbitol derivatives as prepared by adding an alkylene oxide such as ethylene oxide, propylene oxide or the like to sorbitol, polyalcohols such as pentaerythritol and their derivatives, PO/EO random copolymers, etc. It is desirable that the plasticizer is added to PVA in a ratio falling between 1 and 30% by weight, preferably between 2 and 20% by weight.
Preferably, at least one plasticizer selected from sorbitol-alkylene oxide adducts, polyglycerin-alkyl monocarboxylates and PO/EO random copolymers is added to PVA in a ratio falling between 1 and 30% by weight, more preferably between 2 and 20% by weight. Especially preferred are sorbitol-ethylene oxide (1 to 30 mols) adducts.
The fibers having been spun out through the spinning nozzle are directly wound up at a high take-up speed without being drawn, but if desired, they are drawn. The fibers may be drawn to a draw ratio of (elongation at break (HDmax)xc3x970.55 to 0.9) at a temperature not lower than the glass transition point (Tg) of PVA.
If the draw ratio is smaller than HDmaxxc3x970.55, fibers having high strength could not be obtained stably; but if larger than HDmaxxc3x970.9, the fibers will become readily broken. Regarding the drawing mode, the fibers having been spun out through the spinning nozzle are once wound up and then drawn, or are directly drawn immediately after having been spun. In the invention, the fibers may be drawn in any mode of the two. While being drawn, in general, the fibers are heated, for which any of hot air, hot plates, hot rolls, water bathes and the like are employable.
As a rule, the drawing temperature may be around Tg of the polymer constituting the fibers when the crystallized part of the non-drawn fibers is small. However, the polyvinyl alcohol for use in the invention crystallizes rapidly, and therefore the non-drawn fibers of the polymer rapidly crystallize to a relatively high degree. Accordingly, at around Tg of the polymer, the crystallized part of the non-drawn fibers could hardly undergo plastic deformation. For these reasons, even when the non-drawn fibers of the invention are drawn in a mode of contact heat drawing with, for example, hot rollers or the like, the drawing temperature for them shall be relatively high (for example, falling between 70 and 120xc2x0 C. or so). On the other hand, when they are drawn under heat by the use of a non-contact heater such as a heating tube or the like, it is desirable that the drawing temperature for them is much higher than the above, for example, falling between 150 and 200xc2x0 C. or so.
If the fibers are drawn at a temperature not lower than the glass transition point of the polymer constituting them but to a draw ratio overstepping the defined range of (elongation at break (HDmax)xc3x970.55 to 0.9), the drawn fibers shall have streaky recesses running longitudinally on the surface thereof in the direction of the fiber axis. In that condition, when the drawn fibers having such streaky recesses are processed, woven or knitted in the subsequent steps, the recesses will be fibrillated to give scum while the fibers are pressed against guides and others or they receive some friction power applied thereto in the subsequent steps. The fibril scum often contaminates the woven or knitted fabrics to make the fabrics have defects, or often breaks the fibers being processed, woven or knitted. Therefore, drawing the fibers under the condition overstepping the defined range as above is unfavorable. In the present invention, the polyvinyl alcohol fibers are drawn under the condition falling within the define range as above, and therefore, the drawn fibers are substantially free from streaky recesses having a length of 0.5 xcexcm or more and running longitudinally on the surface in the direction of the fiber axis. The drawn fibers of the invention are therefore characterized in that they are neither fibrillated nor broken in the subsequent steps of processing, weaving or knitting them. As opposed to these, PVA fibers produced in the conventional wet-spinning method, dry-jet-wet-spinning method, dry-spinning method or gel-spinning method have many streaky recesses running on the entire surface thereof In the direction of the fiber axis. In fact, in the conventional spinning methods, it is extremely difficult to produce PVA fibers free from such streaky recesses having a length of 0.5 xcexcm or more.
The streaky recesses referred to herein are meant to indicate thin and long recesses formed on the surface of fibers, and they have a length of 0.5 xcexcm or more and run longitudinally almost in the direction of the fiber axis. The rough structure of the fiber surface with such streaky recesses thereon can be seen by magnifying the fiber surface to 2,000 to 20,000 times with a scanning electronic microscope. As so mentioned hereinabove, the streaky recesses are almost inevitable in the conventional spinning technique of wet-spinning, dry-jet-wet-spinning, dry-spinning, gel-spinning and the like. Even in a melt-spinning method, fibers drawn to a high draw ratio to have an increased degree of orientation will often have such streaky recesses on their surface.
The cross-section profile of the fibers of the invention is not specifically defined. Being different from fibers produced through wet-spinning, dry-spinning or dry-jet-wet-spinning, the fibers of the invention are produced in any ordinary melt-spinning method, and may have any desired cross-section profile including circular, hollow or modified cross sections, depending on the shape of the spinning nozzle used. In view of the process compatibility in producing and processing the fibers and in weaving or knitting them into fabrics, it is desirable that the fibers of the invention have a circular cross-section profile.
As a rule, an oil is applied to spun fibers. Since the fibers of the invention are soluble in water and have high moisture absorbability, it is desirable to apply a water-free, straight oil to them.
The oil generally comprises a water-free antistatic component and a leveling component. For example, it may comprise any one or more selected from polyoxyethylene lauryl phosphate diethanolamine salts, polyoxyethylene cetyl phosphate diethanolamine salts, alkylimidazolium ethosulfates, cationated derivatives of polyoxyethylene laurylaminoethers, sorbitan monostearate, sorbitan tristearate, polyoxyethylene sorbitan monostearates, polyoxyethylene sorbitan tristearates, stearic acid glycerides, polyoxyethylene stearylethers, polyethylene glycol stearates, polyethylene glycol alkyl esters, polyoxyethylene castor waxes, propylene oxide/ethylene oxide (PO/EO) random ethers, PO/EO block ethers, PO/EO modified silicones, cocoyldiethanolamides, polymer amides, butyl cellosolve, mineral oils, neutral oils.
For applying the oil to the fibers, employable is any ordinary method using a contact roller or a drawing pen.
The take-up speed for the fibers varies, depending on the mode of forming the fibers. For example, the fibers are produced in a process comprising once winding up the spun fibers followed by drawing them; or a direct drawing process where the fibers are spun and immediately drawn in one step; or a non-drawing process where the fibers are spun at a high speed and directly wound up without being drawn. In any of these processes, in general, the fibers are taken up at a take-up speed falling between 500 m/min and 7000 m/min. The take-up speed for the fibers is much higher than that for fibers produced in the conventional wet-spinning, dry-jet-wet-spinning or dry-spinning method. That is, the fibers of the invention can produced at such an extremely high speed. Needless-to-say, the fibers can be produced at a take-up speed lower than 500 m/min, but such a low take-up speed is meaningless for the fibers from the viewpoint of the productivity. On the other hand, however, at a too high take-up speed over 7000 m/min, the fibers will be cut or broken.
The water-soluble PVA fibers of the invention can be controlled for their shrinkage profile in water by controlling the conditions for producing them. Where the fibers are intended not to shrink or to shrink only a little while they are in water, they are preferably subjected to heat treatment. The heat treatment may be effected along with or separately from the drawing treatment in the process where the spun fibers are drawn.
Where the fibers are subjected to the heat treatment at high temperatures, the maximum degree of shrinkage of the fibers being dissolved in water may be lowered. However, the fibers having undergone heat treatment at high temperatures will often require high dissolution temperature in water. Therefore, it is desirable to define the heat treatment conditions in consideration of the use of the fibers and of the balance between the dissolution temperature in water and the maximum degree of shrinkage of the fibers being dissolved in water. In general, the temperature for the heat treatment preferably falls between the glass transition point of PVA and (Tmxe2x88x9210)xc2x0 C.
If the heat treatment temperature is lower than Tg, the fibers could not well crystallize to a satisfactory degree, and they will much shrink when they are formed into fabrics and subjected to heat-setting treatment. If so, in addition, the maximum shrinkage of the fibers being dissolved in hot water will be over 70%, and, as the case may be, the fibers will absorb much moisture and will be glued together while stored. On the other hand, if the heat treatment temperature is higher than (Tmxe2x88x9210)xc2x0 C., the fibers will be unfavorably glued together when heated.
The drawn fibers may be subjected to the heat treatment while being shrunk. The fibers having undergone the heat treatment while being shrunk could have a reduced degree of shrinkage when they are dissolved in water. The degree of shrinkage to be applied to the fibers being subjected to the heat treatment preferably falls between 0.01 and 5%, more preferably between 0.1 and 4.5%, even more preferably between 1 and 4%. If the degree of shrinkage applied to them is lower than 0.01%, it is substantially ineffective for reducing the maximum degree of shrinkage of the fibers being dissolved in water. However, if the degree is larger than 5%, the fibers being shrunk at such a high degree will be loosened and stably shrinking the fibers will be impossible.
Since PVA for use in the invention is easily soluble in water, it is desirable that the PVA fibers are heat-drawing contacting to a hot plate or the like or heat-drawing in hot air or the like in which they are influenced little by water. If the PVA fibers are inevitably obliged to be drawn in a water bath, it is desirable that the temperature of the water bath is controlled to be not higher than 40xc2x0 C.
Regarding the temperature of water in which the fibers are dissolved and the maximum degree of shrinkage of the fibers being dissolved in water, it is desirable, though depending on the use of the fibers, that the fibers are dissolved in water at low temperatures and the degree of shrinkage of the fibers being dissolved in water is small, in view of the economical aspect and the dimension stability of the fibers. The water dissolution temperature is meant to indicate the temperature to be measured as follows: The fibers are hung in water with a load of 2 mg/denier being applied thereto, and heated herein, and the temperature at which the fibers have broken in water is read. This is the temperature at which the fibers tested dissolve in water. On the other hand, the highest degree of shrinkage of the fibers just before dissolved in this test is read, and this is the maximum degree of the fibers having dissolved in water.
The PVA fibers of the invention are xe2x80x9csoluble in waterxe2x80x9d, and this means that the fibers dissolve in water in the test method as above, irrespective of the time taken until the fibers are dissolved.
In the invention, it is possible to produce water-soluble PVA fibers capable of dissolution in water at a temperature falling between about 10xc2x0 C. and 100xc2x0 C. or so, by varying the type of PVA to be used and the conditions for producing the PVA fibers. However, fibers capable of dissolving in water at low temperatures will easily absorb moisture, and their strength is often low. Therefore, in order to make the fibers have a good balance of all characteristics including easy handlability, practicability and solubility in water, it is desirable that the temperature at which the fibers degrade in water is not lower than 40xc2x0 C.
The temperature at which the water-soluble fibers are processed for dissolving them may be suitably determined, depending on the temperature at which the fibers degrade and on the use of the fibers. As a rule, the processing time may be shorter when the processing temperature is higher. Where the fibers are processed in hot water, the temperature of the water is preferably not lower than 50xc2x0 C., more preferably not lower than 60xc2x0 C., even more preferably not lower than 70xc2x0 C., most preferably not lower than 80xc2x0 C. The treatment for dissolving the melt-spun fibers comprising PVA may be accompanied by decomposition of the fibers.
As the aqueous solution in which the PVA fibers are processed, generally employed is soft water, but any others such as an aqueous alkaline solution, an aqueous acidic solution and the like are also employable. The solution may contain a surfactant and a penetrant.
The maximum degree of shrinkage of the PVA fibers being dissolved in water is preferably at most 70%, more preferably at most 60%, even more preferably at most 50%, further preferably at most 40%, most preferably at most 30%. If the maximum degree of shrinkage of the PVA fibers is too large, the PVA fibers will shrunk too much, for example, when they are formed into fabrics along with other low-shrinkage synthetic fibers and the fabrics are processed in water to dissolve the PVA fibers therein. As a result, the fabrics thus processed will be warped, deformed or wrinkled, and will lose their good shape.
The PVA fibers produced in the manner mentioned above can be formed into various fibrous structures such as yarns, woven fabrics, knitted fabrics and others, either alone or as combined with any other water-insoluble fibers or hardly water-soluble fibers of which the solubility in water is lower than that of the PVA fibers. In those fibrous structures, the PVA fibers may be conjugate fibers or mixed spun fibers comprising PVA and any other thermoplastic polymers.
The PVA fibers of the invention can be used in different modes with no specific limitation, depending on their applications. For example, in one mode of using them, the PVA component may be positively left in the fibrous structures comprising them so that it serves as a binder component therein; and in another mode, the fibers comprising PVA as at least one component are combined with water-insoluble fibers to fabricate structure-modified yarns, mixed filament yarns, spun yarns and other yarns, then the yarns are formed into woven or knitted fabrics, and the fabrics are thereafter processed in water to dissolve and remove the PVA component therein, thereby forming some voids in the final products. In the latter case, the final products produced could have additional functions, and their feel could be improved. For example, they will be bulky, soft to the touch and flexible, and have heat-insulating capability. Regarding the latter case of making fibrous products have some additional functions, for example, polyester fibers or multi-component fibers comprising polystyrene as one component may be processed with an aqueous alkaline solution or an organic solvent so as to attain the intended object. In the invention, however, the fibrous structures processed with harmless water could have additional functions, and this is one characteristic feature of the invention.
The non-woven fabric of the invention comprises the fibers having, as at least one component, the modified polyvinyl alcohol (modified PVA). To fabricate it, any method is employable. For example, the fibers produced in the manner mentioned above may be formed into card webs; or the fibers are, just after having been prepared through melt-spinning, directly formed into non-woven fabrics, for example, in a spun-bonding mode or a melt-blowing mode.
The non-woven fabric may be composed of fibers of the modified PVA alone or of multi-component fibers comprising, as one component, the modified PVA and, as another component, a different water-insoluble or hardly water-soluble thermoplastic polymer of which the solubility in water is lower than that of the modified PVA and which has a melting point of not higher than 270xc2x0 C. In the multi-component fibers, the type of the different thermoplastic polymer may be the same as that mentioned hereinabove for multi-component fibers.
Regarding the cross-section profile of the fibers constituting the non-woven fabric, the fibers are not limited to those having a circular cross section, but include others having various modified cross sections or having a hollow cross section.
The method of producing melt-blown non-woven fabrics comprising the modified PVA is described concretely. In the method, employable are any known melt-blowing devices such as those shown, for example, in Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346, 1956. Briefly, PVA pellets are melted and kneaded in a melt extruder, and the resulting polymer melt is metered with a gear pump, introduced into the spinning nozzle of a melt-blowing device, and spun out through it into fibers while being blown by a hot air stream, then the thus-blown fibers are sheeted on a collector to form a non-woven fabric, and finally, the thus-sheeted non-woven fabric is wound up.
If desired, a cold air stream at a temperature not higher than around 40xc2x0 C. may be applied to the melt-blown fibers just below the nozzle, whereby the adhesion of fibers in the non-woven fabric could be minimized. In this manner, the non-woven fabric produced could be softer.
In the method of producing the non-woven fabric of the invention in the melt-blowing manner as above, it is important that the blowing temperature is controlled to fall between (Tm+10xc2x0 C.) and (Tm+80xc2x0 C.). If the blowing temperature is lower than (Tm+10xc2x0 C.), the melt viscosity of the polymer is too high and the polymer could not form thin fibers even when the blowing air is applied thereto at a high speed. If so, the non-woven fabric produced will have an extremely rough texture. On the other hand, if the blowing temperature is higher than (Tm+80xc2x0 C.), the polymer PVA will pyrolyze and could not be stably spun into fibers.
If desired, the fibers to constitute the melt-blown non-woven fabric of the invention may be partly or wholly pressed under heat to thereby enhance the fiber-to-fiber adhesiveness in the fabric. In that condition, the fabric could have increased strength. The fibers constituting the melt-blown non-woven fabric of the invention poorly adhere to each other when they are formed into webs. Therefore, the fibers forming the webs are often pulled away and the fabric will be thereby broken. To solve the problem, the fibers are partly or wholly pressed under heat so as to be firmly fixed together, for example, through thermal embossing or thermal calendering, and the web strength is thereby increased. With the increased strength, the practical applicability of the fabric can be expanded. In the thermal pressure treatment, the temperature of the hot roll to be used, the pressure, the processing temperature and the pattern of the embossing roll to be used may be suitably determined, depending on the object of the treatment.
The PVA fibers constituting the non-woven fabric of the invention are active to water, and their apparent melting point will lower in the presence of water. Therefore, when the fabric is subjected to the thermal pressure treatment after water is applied thereto, the temperature of the hot roll to be used may be lowered.
The melt-blown non-woven fabric of thermoplastic PVA produced in the manner mentioned above may have a different degree of air-permeability. For example, it may have a degree of air-permeability of from 1 to 400 cc/cm2/sec or so. However, if the mean diameter of the fibers constituting the non-woven fabric is larger than 20 xcexcm, the fabric could not have a degree of permeability falling within that range. Therefore, it is desirable that the mean diameter of the fibers constituting the non-woven fabric is at most 20 xcexcm.
The non-woven fabric of the invention dissolves or swells in water or absorbs water, as having high affinity for water. For example, it well dissolves even in cold water at 5xc2x0 C., and therefore can be processed with water in an ordinary environmental temperature range falling between 5xc2x0 C. and 30xc2x0 C.
The melt-blown non-woven fabric produced in the manner as above is able to dissolve or disintegrate in water. However, if the fabric is desired to be able to dissolve or disintegrate in water at higher temperatures, it may be subjected to additional heat treatment. The heat treatment promotes the crystallization of the resinous fibers constituting the fabric. The heat treatment may be effected in the course of the process of producing the melt-blown non-woven fabric, or may be effected after the fabric produced has been wound up. The non-woven fabric having undergone the heat treatment is not dissolved in water at temperatures lower than 50xc2x0 C., still keeping a degree of PVA weight retentiveness of at least 99% therein, but is rapidly dissolved in water at higher temperatures of, for example, 70xc2x0 C. or higher. Thus, the heat-treated fabric has temperature-dependent degradability in water.
It is important that the non-woven fabric is subjected to the heat treatment at a temperature falling between 40xc2x0 C. and Tmxe2x88x925xc2x0 C.
If the heat-treatment temperature is lower than 40xc2x0 C., the fibers constituting the fabric could not well crystallize to a satisfactory degree at such low temperatures, and such low-temperature heat treatment is ineffective for improving the fabric to make it have the intended temperature-dependent degradability in water. On the other hand, if he heat-treatment temperature is higher than Tmxe2x88x925xc2x0 C., the fibers constituting the fabric will be glued together through such high-temperature heat treatment, and the fabric will have a rough and hard feel, losing a soft touch, and is unfavorable.
The heat treatment may be effected in any desired manner, except the method of directly exposing the non-woven fabric to water in a water bath. For example, the non-woven fabric maybe heated in hot air, or by the use of hot plates, hot rollers, etc. Preferably, it is heated with hot rollers with which continuous heat treatment is possible on an industrial scale. In the method of heating the non-woven fabric with such hot rollers, the non-woven fabric is kept in direct contact with hot rollers. In the heat treatment, one or both surfaces of the non-woven fabric may be heated. If desired, the non-woven fabric may be heated under pressure.
The temperature at which the non-woven fabric is dissolved disintegrated in water may be varied by varying the formulation of the polymers to form the fabric, and also by varying the fiber-blowing conditions including the temperature, the flow rate of the blowing air, etc., as well as the heat history for the post-heat-treatment of the fabric including the heat-treatment temperature and time, etc. The non-woven fabrics thus produced and processed under different conditions could have varying temperature-dependent degradability in water. For example, some are degradable in cold water, but some others are degradable only in boiling water.
The temperature of water when the non-woven fabric is dissolved or disintegrated may be suitably determined, depending on the use of the fabric. As a rule, the processing time may be shorter when the processing temperature is higher. Where the fabric is dissolved or disintegrated in hot water, the temperature of the water is preferably not lower than 50xc2x0 C., more preferably not lower than 60xc2x0 C., even more preferably not lower than 70xc2x0 C., most preferably not lower than 80xc2x0 C. The treatment for dissolving or disintegrate the melt-blown non-woven fabric comprising PVA may be accompanied by decomposition of the fibers constituting the fabric.
PVA for use in the invention is biodegradable, and, when processed with activated sludge or buried in the ground, it is degraded to give water and carbon dioxide. When its aqueous solution is continuously processed with activated sludge, PVA is almost completely degraded within 2 days to one month. In view of its biodegradability, it is desirable that PVA for fibers in the invention has a degree of saponification falling between 90 and 99.99 mol %, more preferably between 92 and 99.98 mol %, even more preferably between 93 and 99.97 mol %. It is also desirable that PVA for them has a 1,2-glycol bond content falling between 1.2 and 2.0 mol %, more preferably between 1.25 and 1.95 mol %, even more preferably between 1.3 and 1.9 mol %.
PVA having a 1,2-glycol bond content of smaller than 1.2 mol % has poor biodegradability and, in addition, its spinnability will be often poor as its melt viscosity is too high. On the other hand, PVA having a 1,2-glycol bond content of larger than 2.0 mol % has poor heat stability, and its spinnability will be often poor.
The 1,2-glycol bond content of PVA can be obtained from the peak appearing in NMR. Briefly, PVA is saponified to have a degree of saponification of at least 99.9 mol %, then fully washed with methanol, and dried at 90xc2x0 C. under reduced pressure for 2 days. This is dissolved in DMSO-D6, to which are added a few drops of trifluoroacetic acid. The resulting sample is subjected to 500 MHz proton NMR (with JEOL GX-500) at 80xc2x0 C. From the NMR data, obtained is the 1,2-glycol bond content of PVA.
Concretely, the methine-derived peaks for the vinyl alcohol units in PVA are assigned to the region falling between 3.2 and 4.0 ppm (integrated value A); and the methine-derived peak for one 1,2-glycol bond therein is to 3.25 ppm (integrated value B). The 1,2-glycol bond content of PVA is calculated as in the following formula in which A indicates the degree of modification (mol %).
1,2-Glycol bond content (mol %)=100B/{100A/(100xe2x88x92xcex94)}
The fibers of the invention that comprise PVA as at least one component, and also the fibrous structures containing the fibers of the invention, such as yarns, knitted or woven fabrics, non-woven fabrics and others have many applications including, for example, binder fibers for papermaking, binder fibers for non-woven fabrics, staples for dry-process non-woven fabrics, staples for spinning, multi-filaments for knitted and woven fabrics (structure-modified yarns, mixed filament yarns), base fabrics for chemical lace, woven fabrics for robes, sewing threads, water-soluble wrapping materials; sanitary materials such as diaper liners, paper diapers, sanitary napkins, pads for incontinence, etc.; medical supplies such as surgical gowns, surgical tapes, masks, sheets, bandages, gauze, clean cotton, base fabrics for first-aid adhesive tapes, base fabrics for plasters, wound covers, etc.; wrapping materials, splicing tapes, hot-melt sheets (including temporary tacking sheets), interlinings, sheet for planting, covers for agricultural use, sheets for protecting roots, water-soluble ropes, fishing lines, reinforcing materials for cement, reinforcing materials for rubber, masking tapes, caps, filters, wiping cloths, abrasive cloths, towels, small damp towels, cosmetic puffs, cosmetic pads, aprons, gloves, table cloths; various covers such as toilet seat covers, etc.; wall cloths; air-permeable, re-wettable adhesives to be used as lining pastes for wallpapers, wall cloths, etc.; water-soluble toys, etc.
The invention is described concretely with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. Unless otherwise specifically indicated, parts and % referred to in the following Examples are all by weight.
Analysis of PVA:
Unless otherwise specifically indicated, PVA was analyzed according to JIS-K6726.
To measure its degree of modification, a modified polyvinyl ester or modified PVA was subjected to 500 MHz proton NMR (with JEOL GX-500).
The alkali metal ion content of PVA was obtained through atomic absorptiometry.
Solubility in Water:
The temperature at which the PVA fibers of the invention are dissolved in water, the water dissolution temperature, was measured as follows:
With a load of 2 mg/denier being applied to them, the fibers were immersed in water along with a graded scale. The depth of the fibers being immersed in water was about 10 cm. With the fibers being immersed therein in that condition, water was heated from 20xc2x0 C. up to the temperature at which the fibers began to break by dissolution, at a heating rate of 1xc2x0 C./min. The temperature at which the fibers immersed in water began to break was read. While being heated so, the length of the fibers was read with the scale until the fibers broken by dissolution. Based on the change in the length of the fibers, the maximum degree of shrinkage of the fibers was obtained. Apart from this, the fibers were stirred in water at 90xc2x0 C. for 1 hour, and macroscopically checked for the presence or absence of any insoluble in water.
Strength and Elongation of Fibers:
Measured according to JIS L1013.
Spinnability:
PVA was melt-kneaded in a melt extruder, the polymer melt stream was introduced into a spinning head, and metered with a gear pump. For single-component spinning, used was a nozzle with 24 orifices each having a diameter of 0.25 mm; but for multi-component spinning, used was a nozzle with 24 orifices each having a diameter of 0.4 mm. The polymer melt was spun out through the nozzle, and wound up at a rate of 800 m/min. This spinning test was continued for 6 hours. During the test, the condition of the spun fibers was all the time checked, and the spinnability of the polymer tested was evaluated as follows:
OO: Not broken at all, the spun fibers were all wound up continuously for 6 hours.
O: The spun fibers were broken once for 6 hours, but could be wound up for 6 hours as multi-filaments.
Oxcx9cxcex94: The spun fibers were broken twice or more for 6 hours, but could be wound up for 6 hours as multi-filaments.
xcex94: The spun fibers were much broken, and could be wound up only for about 5 minutes as multi-filaments.
x: The spun fibers were much broken and could not be wound up at all.
Degradability of Non-woven Fabrics in Water:
The degradation of the present non-woven ranges from its complete dissolution in water to its partial disintegration in water.
About 0.1 g of a square sample was cut out of a non-woven fabric to be tested, and its weight was measured. This was put into 1000 cc of distilled water having been controlled to have a predetermined temperature, and kept therein for about 30 minutes with intermittently stirring it. Then, the condition of the sample was observed. When the sample was seen to have lost the structure as a non-woven fabric by dissolution or disintegration, it was defined as xe2x80x9cdegradedxe2x80x9d.
When the sample was shrunk, swollen or warped to be in a clump, and it was impossible to macroscopically judge as to whether or not it could keep the structure of the non-woven fabric, then the sample was taken out of water. This was dried, and thereafter its weight was measured. The weight thus measured was compared with the original weight of the sample. When the weight retentiveness of the sample was at least 70%, the sample was considered as xe2x80x9cdegradedxe2x80x9d.
Weight Retentiveness of Non-woven Fabrics:
The weight retentiveness of the non-woven fabric of PVA was determined as follows:
The original weight of the non-processed fabric (this was left at 25xc2x0 C. at 60% RH for 24 hours) was measured. The fabric was immersed in water at 50xc2x0 C. for 30 minutes, then taken out of it, dried, and thereafter kept at 25xc2x0 C. at 60% RH for 24 hours, and its weight was measured. The weight retentiveness of the fabric is represented by weight percentage of the weight of the processed fabric to the original weight of the non-processed fabric.
Strength and Elongation of Non-woven Fabrics:
Measured according to the xe2x80x9cnon-woven interlining test methodxe2x80x9d in JIS L1085.
Air Permeability of Non-woven Fabrics:
Measured according to the Method A of the xe2x80x9cgeneral fabric test methodxe2x80x9d in JIS L1096, for which was used a Frazier tester. Mean Diameter of Fibers Constituting Non-woven Fabrics:
With a scanning electronic microscope, pictures (xc3x971000) of the non-woven fabric to be measured were taken, showing the surface of the fabric. Two diagonal lines were drawn on each picture, and the thickness of the fibers crossing the lines was measured. From the magnification of the microscope used, the data were converted into the actual diameter of each fiber. 100 fibers were measured and averaged to obtain the mean diameter of the fibers constituting the fabric.
On the pictures, unclear fibers and overlapped fibers, of which the diameter of one fiber could not be measured, were omitted.